There are approximately one thousand operational satellites in orbit today. Five hundred of these satellites are in either a Low-Earth Orbit (LEO) or a geostationary orbit. The usefulness of satellites critically depends on their ability to communicate with their associated Near Earth Networks (NEN). With technological advances in sensing and image capture, the manufacturers of communication links between earth and satellites have been under pressure to provide higher bandwidths at a reduced weight and reduced dissipated power. The weight directly influences the price of a satellite launch, with present day prices ranging in the $10,000 to $100,000/kg, while radiating heat into space requires radiant cooling elements which require space on the spacecraft, ultimately adding to its weight. The satellite thermal design challenge is therefore obtaining more RF power from the transmitter with the same radiant cooling area or reducing the size of the cooling elements while still delivering equal or higher RF power from the transmitter.
There is a growing interest in low earth orbit (LEO) satellites with a small form factor. Due to their smaller size and weight (less than 500 kg), femto-satellites, pico-satellites, micro-satellites and mini-satellites generally cost less to build and deploy into orbit above the Earth than standard, large satellites. CubeSats or U-class spacecraft present opportunities for educational institutions, governments, and commercial entities to launch and deploy satellites for a variety of purposes with fewer costs compared to traditional, large satellites. CubeSats are one type of miniaturized satellite for space research, made up of multiples of 10 cm×10 cm×11.35 cm cubic units with mass not greater than 1.33 kg; these satellites often use commercial off-the-shelf components.
Thermal management in satellites comprises balancing the positive energy flux absorbed from the sun (radiative heating and energy converted by the solar cells) against the negative energy fluxes in the form of heat dissipated by radiation (majority) and in the RF and optical signals emitted by the satellite communication systems (minority).
A simplified block diagram of the main components of a satellite is shown in FIG. 1. A satellite comprises an energy supply block which comprises solar cells and batteries 106, waste heat radiators 107, the control/communications block 101, and the payload 102, which may include sensing, measurement or imaging instrumentation. The control/communications block 101 comprises a transmitter 103, a receiver 104, and a control unit 105 to manage the satellite and the communications to Earth. From a thermal management point of view, not counting the variable energy needed and dissipated in the payload 102, the power PTX dissipated in the transmitter 103 is typically greater than the sum of the powers dissipated in the control unit 105 PCONT and the receiver unit 104 PRX. In other words, PTX>PRX+PCONT, and this is especially true for satellites with communications in the high frequencies (e.g. K-band). The energy consumption (and subsequent required heat dissipation) in the transmitter PTX increases with the data transfer rate according to the Shannon-Hartley theorem. The key thermal-design challenge in satellites is the unbalanced distribution of thermal loads caused by internal and external heat sources. The minimum size of the satellite is determined by the transmitter's dimensions and whether the satellite can produce sufficient power to run the transmitter. This in turn determines the link budget.
Satellite thermal management involves redistributing heat between different subsystems by way of heat conduction (contact) and by radiative emission of thermal energy to space via any surface of the satellite, including specially designed surfaces that have been coated and otherwise prepared for the most efficient radiation of heat into space. Such surfaces are referred to as heat radiators or radiant cooling elements. A heat radiator is a surface on the satellite specifically built for radiating heat into space. These radiators come in several different forms, such as, spacecraft structural panels, flat-plate radiators mounted to the side of the spacecraft, and panels deployed after the spacecraft is on orbit. Whatever the configuration, all radiators reject heat by infrared (IR) radiation from their surfaces. The radiating power depends on the surface's emittance and temperature of the radiant cooling element. The rate of heat radiation increases with the temperature of the radiator. The radiator must reject both the spacecraft waste heat and any radiant-heat loads from the environment. Most radiant cooling elements are therefore given surface finishes with high IR emittance to maximize heat rejection and low solar absorptance to limit heat from the sun. If an existing structural panel is used as a radiant cooling element, there is no additional weight associated with a heat radiator, while heavy deployable radiators can weight as much as 12 kg/m2. Geostationary satellites always keep one side turned to the sun, with the other side being used to dissipate heat. In small satellites, all sides are covered with solar cells and little or no surface is left for heat dissipation. This limits the amount of power dissipation possible.
The temperature of space is ˜2.7 K, but due to heat absorbed from the sun and electrical energy dissipated by the electronics in a satellite the operating temperatures of the satellite electronics are significantly higher. The temperature of operation of the transmitter subsystem ranges from 240 K to 350 K.
It is clear that there is a need in the industry for communication modules which enable more efficient heat conduction and dissipation into space. This can be most readily achieved by developing a high power RF power amplifiers with improved thermal management which will allow either more RF power for the same radiant cooling element surface or smaller radiant cooling element area being sufficient for equal or smaller radiant cooling elements.